1. Field of the Invention
The present invention relates to a method to improve Quality of Service (QoS) in a wireless network, preferably a WLAN according to the IEEE proposal 802.11e with an Access Point (AP) and at least one mobile station (STA). Different Access Categories (AC) are defined for the transmission of frames, wherein the frames are transmitted according to their priority. A Power Save Mode (PSM), preferably one according to the IEEE standard 802.11, is used in the network.
2. Description of the Related Art
Wireless networks—Wireless Local Area Networks (WLANs)—are a direct extension of wired LANs and are meanwhile widely used due to the development of the IEEE 802.11 standard family. WLANs include an Access Point (AP) as well as in general several stations (STA) with the transmission of data between the AP and the single stations being realized via a radio link. Such kinds of networks are widely used in so-called hot spot areas and offer a broadband Internet access, for example in offices, airports, hotels or conference centers.
The integration of WLANs and 3G-systems, which are currently developed intensively, offer some further interesting applications. To achieve a smooth interworking of both technologies, several prerequisites must be fulfilled though. One of the key conditions is for example the provision of guarantees of a comparable Quality of Service (QoS) for both systems, so basic services such as voice over IP (VOIP) for example, are supported. The IEEE 802.11e MAC (Medium Access Control) extension provides already functionalities to support QoS in a WLAN. In devices having 3G and WLAN-capabilities, the IEEE 802.11e proposal is therefore implemented, so applications requiring QoS guarantees, such as VoIP for example, will be supported. See IEEE 802.11 WG, IEEE 802.11e/D13.0, “Draft Supplement to Standard for Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Part 11: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications: Medium Access Control (MAC) Enhancements for Quality of Service (QoS),” January 2005.
When introducing wireless LAN technology into mobile devices, such as mobile phones for example, the limitations of mobile devices which are in general battery operated are to be taken into consideration. Due to the fact that transmission is performed in a WLAN over a commonly shared medium, data frames can arrive at their destination at any time, and therefore WLAN devices usually operate at full power.
In contrast, devices with restrictions due to battery reasons cannot run constantly at full power and use therefore energy saving mechanisms. In the IEEE 802.11 standard, a power save mode (PSM) is specified which can be used in such devices. See IEEE, “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications,” IEEE Standard 802.11, June 1999. In this context, problems occur with the interaction of IEEE 802.11-PSM and the 802.11e QoS mechanisms.
In the 802.11e proposal, a differentiation of the (data) traffic is defined which is based on the different priorities of the frames to be sent. The differentiation is achieved by varying different parameters. In this way, the time period a station has to identify the wireless channel in a network as being free and before starting the so-called back-off or the transmission, can be varied depending on the respective priority. Furthermore, a variation of length of the so-called Contention Window (CW) used for the back-off and a variation of the duration during which a station is allowed to send after having got access to the channel is provided.
This is realized by the introduction of four Access Categories (ACs) being assigned different priorities. In detail, these are the categories AC_VO, AC_VI, AC_BE and AC_BK with the extensions labeling the applications the respective category is intended for. VO represents Voice (highest priority), VI is video, BE is Best Effort, and BK means Background (lowest priority). The different AC queues are —depending on their priority—emptied at different rates. As each AC has its own queue, the rate at which an AC is worked on has no influence on the rest of the ACs.
FIG. 1 shows schematically in a diagram the standard storage mechanism as used in case that the Power Save Mode of standard 802.11 and the QoS extension 802.11e are used simultaneously. The implementation of two ACs is shown, with AC_VO representing a high priority class (voice) and AC_BE representing a low priority class (Best Effort). Incoming data frames are first buffered in the common PSM, no matter what priority they have. As soon as a Power Save (PS) poll request of a station is received by the AP, the corresponding frame leaves the common PSM queue and the frame undergoes a ToS (Type of Service) check. Depending on the priority of the frame, it is sorted into one of the queues, i.e. in the shown embodiment either in the AC_VO queue or in the AC_BE queue.
However, problems occur if the aforesaid QoS differentiation mechanisms are used together with the 802.11-PSM. In such a case, usually all the frames of different access categories are buffered at the Base Station or at the Access Point in the same queue—the PSM queue.
Under congestion conditions, the different service rates of the different ACs result in filling up the PSM queue with frames of low priority while frames of high priority are emptied from the PSM queue. If utilizing the normal procedure saying that in case of a full queue newly arriving frames will be dropped, the performance of ACs of high priority will suffer a significant degradation as most of the frames of high priority will directly be discarded because the PSM queue is filled up with frames of low priority.
Due to the service rates which depend on the priorities that are different for the ACs, the PSM queue fills up with frames of low priority. If the buffer of a PSM queue is full, newly incoming frames at the PSM queue are usually discarded. Under congestion conditions, i.e. when the buffer at the PSM queue is full, the service of the AC VO suffers therefore a significant degradation, due to the slower service rate of low priority frames. As a result, the utilization of the standard PSM has a contrary effect regarding the desired functionality of 802.11e.
A possible approach to avoid the above described problem is to modify the default behavior of the queue system. A modification could be done in such a way that not the newly arriving frames are discarded, but that the oldest frames are discarded and replaced by newly arriving frames. This solution would basically enhance the performance of the system but it would not guarantee for all cases that frames of high priority will not be dropped for newer frames of a lower priority.